Engine manufacturers have focused on improving engine design in order to improve fuel economy and efficiency (typically, based on Federal Corporate Average Fuel Economy (CAFE) standards). While improvements in engine design and operation have contributed, improved formulation of engine oil lubricant may also improve fuel economy and efficiency. Lubricants function to reduce and disperse engine deposits which accumulate when the engines are running. They also serve to reduce the friction between sliding moving parts (typically metallic or ceramic) that are in contact.
It is well known for lubricating oils to contain a number of additives (including antiwear agents, antioxidants, dispersants, detergents etc.) used to protect the mechanical devices such as internal combustion engines from wear, oxidation, soot deposits and acidity build up. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). It is believed that ZDDP antiwear additives protect the engine by forming a protective film on metal surfaces. It is believed that ZDDP can have a detrimental impact on fuel economy and efficiency. Consequently, engine lubricants also contain friction modifier to obviate the potential detrimental impact of ZDDP on fuel economy and efficiency. Both ZDDP and friction modifier function by adsorption on sliding surfaces, and each may interfere with each other's respective functions.
Further, engine lubricants containing phosphorus compounds and sulfur have been shown to contribute in part to particulate emissions, and emissions of other pollutants. In addition, sulfur and phosphorus tend to poison the catalysts used in catalytic converters, resulting in a reduction in performance of said catalysts.
With increasing control of emissions (often associated with contributing to NOx formation, SOx formation, formation of sulfated ash and reducing the efficiency of after-treatment catalytic converters) there is a desire towards reduced amounts of sulfur, phosphorus and sulfated ash in engine oils. However, reducing the levels of antiwear additives such as ZDDP, is likely to increase wear and result in other detrimental performance of an engine.
International Publication WO 2005/087904 discloses a lubricant composition containing at least one hydroxycarboxylic acid ester or hydroxy polycarboxylic acid. The lubricant composition disclosed may also contain zinc dihydrocarbyldithiophosphates, or other phosphorous-containing additives such as trilauryl phosphate or triphenylphosphorothionate. The lubricant composition has anti-wear or anti-fatigue properties.
International Publication WO 2006/044411 discloses a low-sulfur, low-phosphorus, low-ash lubricant composition suitable for lubricating an internal combustion engine, containing a tartrate ester, or amide having 1 to 150 carbon atoms per ester of amide group.
U.S. Pat. No. 5,338,470 discloses alkylated citric acid derivatives obtained as a reaction product of citric acid and an alkyl alcohol or amine. The alkylated citric acid derivative is effective as an antiwear agent and friction modifier.
U.S. Pat. No. 4,237,022 discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 4,952,328 discloses lubricating oil compositions for internal combustion engines, comprising (A) oil of lubricating viscosity, (B) a carboxylic derivative produced by reacting a succinic acylating agent with certain amines, and (C) a basic alkali metal salt of sulfonic or carboxylic acid.
U.S. Pat. No. 4,326,972 discloses lubricant compositions for improving fuel economy of internal combustion engines. The composition includes a specific sulfurized composition (based on an ester of a carboxylic acid) and a basic alkali metal sulfonate.
U.S. patent application 60/862,534, now U.S. Publication 2010-0048437, Brown et al., Feb. 25, 2010, discloses malonate esters suitable as antiwear agents.
Canadian Patent CA 1 183 125 discloses lubricants for gasoline engines containing alkyl-ester tartrates, where the sum of carbon atoms on the alkyl groups is at least 8.
Consequently, it would be desirable to provide a lubricating composition capable of providing at least one of (i) reducing or preventing phosphorus emissions, (ii) reducing or preventing sulfur emissions, (ii) wholly or partially replacing ZDDP in lubricating oils, (iii) improving fuel economy, and (iv) fuel economy retention/efficiency. The present invention provides a friction modifier capable of achieving at least one of these objectives. In addition it may also be desirable for friction modifier to not have a detrimental affect on other components of a mechanical device. It may also be desirable for the friction modifier to have antioxidant performance and, optionally, anti-wear performance.